Electrolyte-controlled discharge product distribution of Na-O₂ batteries: a combined computational and experimental...

Tuning the composition of discharge products is an important strategy to reduce charge potential, suppress side reactions, and improve the reversibility of metal-oxygen batteries. In the present study, first-principles calculations and experimental confirmation were performed to unravel the influence of O₂ pressure, particle size, and electrolyte on the composition of charge products in Na-O₂ batteries. The electrolytes with medium and high donor numbers (>12.5) are favorable for the formation of sole NaO₂, while those with low donor numbers (<12.5) may permit the formation of Na₂O₂ by disproportionation reactions. Our comparative experiments under different electrolytes confirmed the calculation prediction. Our calculations indicated that O₂ pressure and particle size hardly affect discharge products. On the electrode, only one-electron-transfer electrochemical reaction to form NaO₂ takes place, whereas two-electron-transfer electrochemical and chemical reactions to form Na₂O₂ and Na₃O₄ are prevented in thermodynamics. The present study explains why metastable NaO₂ was identified as a sole discharge product in many experiments, while thermodynamically more stable Na₂O₂ was not observed. Therefore, to achieve low overpotential, a high-donor-number electrolyte should be applied in the discharge processes of Na-O₂ batteries.